Menaquinones are a family of molecules which, as a whole, constitute the components of vitamin K2.
In their most common oxidized form, menaquinones have general formula (I):

wherein n is 0 or an integer between 1 and 11.
They differ in terms of the number of isoprene units (the part of the molecule shown in brackets in formula I), the number of which can range from 0 to 11, although the most common forms range from 4 to 7 and relate to menaquinone-4 (MK-4), MK-5, MK-6 and MK-7 respectively. Menaquinones MK-4 and MK-7 are the most plentiful, and are the two available on the diet products market.
The main food sources of vitamin K2 are eggs, dairy products, liver and fermented soya.
The therapeutic use of vitamin K, and especially that of vitamin K2, is described in several patents and scientific publications, with applications ranging from the cardiovascular field (M. K. Shea et al., Am. J. Clin. Nutr. (2009) 89: 1-9) to bone metabolism (M. H. J. Knapen et al., Osteoporos Int (2013) 24:2499-2507). More recent applications of vitamin K relate to the field of inflammation (M. K. Shea et al., Am J Epidemiol. (2008); 167(3): 313-320).
However, the intestinal absorption of the various forms of vitamin K following oral intake, either in food or as a diet supplement, is not very high due to the strongly lipophilic nature of the molecule.
The administration of vitamin K2 in its reduced form of menaquinol, a much more hydrophilic molecule, is believed to increase the absorption of the vitamin, as already demonstrated for a structurally similar molecule, ubiquinol, the reduced form of ubiquinone (M. Evans et al., J. Funct. Foods (2009) 1(2); 240).
The menaquinols are represented by the following general formula (II):

wherein N is as defined above.
However, menaquinols are highly unstable and tend to reoxidise very quickly to menaquinones under the effect of atmospheric oxygen or mild oxidizing agents. The stability of the reduced form of vitamin K2 is difficult to guarantee even in an inert oxygen-free environment, as in the case of the isolated dried solid, stored sealed in a nitrogen atmosphere.
Pharmaceutical and nutraceutical compositions containing menaquinol of formula (II) as active ingredient are described in EP 2060256 A1, wherein menaquinol is prepared by reducing a solution of menaquinone in an organic solvent with an aqueous solution of reducing agent, followed by separation of the phases and recovery of menaquinol from the organic phase.